The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the xe2x80x9cimage area,xe2x80x9d generated on hydrophilic regions on a substrate. When the surface of the printing plate is moistened with water and printing ink is applied, hydrophilic regions retain the water and repel the printing ink, and the image area accepts the printing ink and repels the water. The printing ink retained on the image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
Lithographic printing plates typically comprise a radiation-sensitive coating applied over the hydrophilic surface of a substrate. Conventional radiation-sensitive coatings include photosensitive components dispersed within an organic polymeric binder. After a portion of the coating is exposed to radiation (commonly referred to as imagewise exposure), the exposed portion becomes either more developable or less developable in a particular liquid than an unexposed portion of the coating. A printing plate is generally considered a positive-working plate if, after exposure to radiation, the exposed portions or areas of the radiation-sensitive coating become more developable and are removed in the developing process to reveal the hydrophilic surface. Conversely, the plate is considered a negative-working plate if the exposed portions or areas become less developable in the developer and the unexposed portions or areas are removed in the developing process. After being developed in a suitable liquid, the coating areas (i.e. image area) that remain on the plate provide an ink-receptive image, while the revealed regions of the substrate""s hydrophilic surface repel ink.
Radiation exposure of imaging layers is generally performed using either ultraviolet, infrared (xe2x80x9cIRxe2x80x9d) or visible radiation. IR radiation exposure (as well as other types of radiation exposure) may be advantageously utilized in an imaging technique referred to herein as xe2x80x9cdirect-writexe2x80x9d imaging. Direct-write imaging using infrared radiation is a process in which a thermally sensitive coating of a printing plate precursor is exposed to infrared radiation from a laser source. More particularly, a computer-controlled infrared laser imagewise exposes small regions of the thermally sensitive composition to produce an image area pixel-by-pixel. Examples of plates prepared by this process are reported in U.S. Pat. No. 5,372,915 (Haley et al.). These plates include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. Although the reported plates utilize direct writing techniques, the imaged plates must still be developed in an alkaline solution prior to mounting on a press.
It has further been recognized that such direct writing techniques may be utilized in the formation of xe2x80x9cprocesslessxe2x80x9d printing plates. As used herein, the term xe2x80x9cprocesslessxe2x80x9d refers to printing plate precursors that do not require one or more conventional processing steps (e.g. development) prior to mounting on a printing press.
One method for forming processless printing plates is through ablation of a thermally sensitive layer. For example, Canadian 1,050,805 (Eames) reports a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer containing laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). When such plates are exposed to focused, near-IR radiation with a laser, the absorbing layer converts the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are reported in Research Disclosure 19201 (1980) as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoat layer. These plates are developed by wetting with hexane and rubbing. Additional patents reporting ablatable printing plates include U.S. Pat. No. 5,385,092 (Lewis et al.), U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), U.S. Reissue 35,512 (Nowak et al.), and U.S. Pat. No. 5,378,580 (Leenders).
Ablatable printing plates have a number of disadvantages. The process of ablation tends to produce debris and vaporized materials in the image setting equipment, which must consequently be collected. Also, the laser intensity or power required for ablation may be very high, and the components of such printing plates may be expensive, difficult to use, possess a reduced life, and may produce an unacceptable printing quality.
Thermal or laser mass transfer is another method of preparing processless lithographic printing plates. Such methods are reported, for example, in U.S. Pat. No. 5,460,918 (Ali et al.) wherein a hydrophobic image is transferred from a donor sheet to a microporous hydrophilic crosslinked silicated surface of the receiver sheet. U.S. Pat. No. 3,964,389 (Peterson) reports a process of laser transfer of an image from a donor material to a receiver material requiring a high temperature post-heating step.
EP-A 0 652 483 (Ellis et al.) reports processless lithographic printing plates that are imageable using IR lasers, and that do not require wet processing prior to mounting on a press. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups.
U.S. Pat. Nos. 6,482,571 and 6,548,222 to Teng report on-press developable printing plates having a thermosensitive layer including a free radical initiator, a radiation absorbing material and a polymerizable monomer.
More recently, it has been determined that thermally sensitive coatings containing cyanoacrylate polymers may be particularly useful in the formation of processless printing plates. For example, U.S. Pat. No. 5,605,780 (Burberry et al.) reports printing plates that are imaged by an ablation method whereby exposed areas are removed using the heat generated by a focused high-intensity laser beam. The imaging layer is composed of an IR-absorbing compound in a film-forming cyanoacrylate polymer binder. In order for thermal ablation to be successful in such printing plates, the imaging later thickness is generally less than 0.1 xcexcm and the weight ratio of IR-absorbing compound to the cyanoacrylate polymer is at least 1:1. Thus, the imaging layers are quite thin and have a significant amount of expensive IR-absorbing compound.
Additionally, U.S. patent application Ser. No. 09/864,570, filed May 24, 2001 and incorporated herein by reference reports the use of cyanoacrylate polymers in processless printing plates, in which, after exposure to infrared radiation, imaged regions may be developed xe2x80x9con pressxe2x80x9d by contacting an imaged thermally sensitive layer containing the cyanoacrylate polymer with aqueous fount solution.
Although the Burberry patent and U.S. application Ser. No. 09/864,570 report the benefits of using cyanoacrylate polymers in thermally sensitive layers of printing plates (e.g. ink affinity, adhesion, wear characteristics), these reported printing plates polymers may tend to suffer from certain drawbacks. One significant drawback of thermally sensitive layers that include cyanoacrylate polymers is the tendency for the thermally sensitive layer to produce ablative material upon exposure to radiation. Ablation is generally considered undesirable due to the adverse impact it may have on imagesetting equipment
Furthermore, although the U.S. application Ser. No. 09/864,570 reports that conventional top coat layers may be used with thermally sensitive layers containing cyanoacrylates, conventional top coat layers may not perform suitably for use with thermally sensitive layers that include cyanoacrylate particles. More particularly, many conventional top coat layers fail to achieve a suitable balance between ablation reduction and on-press performance. For example top coat layers that are applied in solution with polar or hydrogen-bonding solvents may interact with the thermally sensitive layer to adversely impact coating integrity. More particularly, such solvents may cause the thermally sensitive layer and/or the top coat layer to lack suitable adhesion to adhere to the substrate. Furthermore, such solvents may cause the thermally sensitive layer to wash away during the application of the top coat layer.
Conversely, top coat layers that are applied in solution with non-polar solvents may be difficult to develop on press, which may lead to poor image quality. Other types of top coat layers may not suitably reduce ablation.
Thus, it would be desirable to prepare a top coat layer for application over a polycyanoacrylate-containing thermally sensitive layer that reduces ablation without adversely interacting with the thermally sensitive layer and without adversely affecting image quality.
In one embodiment, the present invention provides a top coat layer for application on a thermally sensitive layer containing polycyanoacrylate particles. The top coat layer also includes polycyanoacrylate particles, but unlike the thermally sensitive layer, the top coat layer is substantially free of photothermal conversion material. The polycyanoacrylate particles may have a major dimension between about 10 and about 600 nm. Optionally, the top coat layer may include a polymeric binder. In one embodiment, the top coat layer is applied onto the thermally sensitive layer described above to form a printing plate precursor. In this embodiment, the thermally sensitive layer includes polycyanoacrylate particles and a photothermal conversion material.
In another embodiment, the present invention provides a method of making a printing plate precursor, in which a top coat coating mixture is applied onto a thermally sensitive layer of a printing plate precursor and is then dried to form a top coat layer on the thermally sensitive layer. The top coat coating mixture generally includes a suitable carrier (e.g. a solvent that does not adversely interact with the thermally sensitive layer), polycyanoacrylate particles mixed with the carrier and optional additives. Suitable non-interactive carriers may include ethyl acetate or n-heptane, alone or in combination with each other and/or with n-propanol. As used herein, the term xe2x80x9ccoating mixturexe2x80x9d refers to any homogeneous or heterogeneous combination or mixture of two or more materials. For example, the coating mixture may be a true solution (i.e. a dispersion at the molecular or ionic level) a dispersion, a colloidal dispersion, a slurry, a suspension, or an emulsion.
Prior to application of the top coat coating mixture, the thermally sensitive layer is applied as a thermally sensitive coating mixture onto the substrate surface, and is then dried to form the thermally sensitive layer. The thermally sensitive coating mixture generally includes a carrier, a cyanoacrylate polymer and a photothermal conversion material. This thermally sensitive carrier may be an aqueous or an organic carrier.
The thermally sensitive layer of the resulting printing plate precursor may be exposed to imagewise radiation such that radiation exposed portions of the thermally sensitive layer have a lower developability in fountain solution than unexposed portions of the layer. The top coat layer may improve ablation during imagewise radiation as compared to printing plate precursors not possessing the top coat layer. Advantageously, the thermally sensitive layer does not require the inclusion of a free-radical initiator.
Advantageously, the imaged printing plate precursor does not need to be developed in aqueous alkaline solution. Instead, the precursor may be developed xe2x80x9con pressxe2x80x9d by fountain solution used as part of the printing process. Such fountain solution is generally an aqueous solution with optional water-miscible organic liquids such as suitable alcohols and alcohol replacements.